Electro-absorption modulators and other modulators have long been used in the processing of optical signals, especially in the telecommunications industry. Conventionally, modulators are embodied in optoelectronic devices that incorporate an optical waveguide. The optical waveguide may be provided in a modulator chip that conventionally includes an optical substrate having a modulation region between two conductive regions. The optical waveguide guides light between the two conductive regions so that a significant amount of optical energy passes through the modulation region. The modulation region has the property that its transparency can be varied by the application of an electric field. Since the modulation region is located between the two conductive regions, a voltage applied between the two conductive regions subjects the modulation region to an electric field. Thus, the applied voltage can control the amount of light passing through the modulator.
In operation of a modulator, data bits may be imposed on the light passing through the optical waveguide of the modulator chip by applying a voltage that has either of two values. One value makes the modulation region more transparent, and the other value makes the modulation region more opaque. Light exiting the modulator chip when the modulation region is more transparent is at a higher optical power than the light exiting the modulator chip when the modulation region is more opaque. The ratio of the optical power of the light exiting the modulator chip in these two states, for given values of voltage, is called the extinction ratio.
The maximum rate at which bits can be imposed on the light is called the bit rate. At high bit rates, the voltage applied to the modulator chip will be varying very rapidly, so the performance of the modulator at high frequencies becomes important. Both high extinction ratio and high bit rate are very desirable modulator properties.
Conventionally, a modulator is driven by connecting an input transmission line (having a characteristic impedance “Z0”) to the modulator, with each of the two conductors of the input transmission line connected to a respective conductive region of the modulator. The modulator is shunted by a load resistor (having a resistance “Rload”=Z0) at the point where the input transmission line connects to the modulator. Unfortunately, this is a non-ideal approximation to the desired matched load situation, because it is a combination of the capacitance of the modulator (“Cmod”) in parallel with the load resistor that terminates the input transmission line, rather than the load resistor alone. As the modulator is operated at higher frequencies, the impedance of this parallel combination will decrease so that the modulator behaves increasingly like a short, because the impedance of capacitance is inversely proportional to frequency. Consequently, at high frequencies, the impedance at the end of the input transmission line will increasingly differ from the characteristic impedance of the input transmission line. This impedance mismatch may result in increased reflection back onto the input transmission line, and decreased signal at the modulator. Both of these effects limit performance of the modulator at high frequency and, accordingly, limit bit rate.
One might attempt to alleviate these limitations on high frequency performance by making the modulator shorter along the direction of the optical waveguide. The parasitic capacitance of the modulator would then be reduced, approximately in proportion to the decrease in length. While this would improve the high frequency performance, it would also decrease the extinction ratio, which is approximately proportional to modulator length. Thus, there is an unfortunate trade-off between these two desirable aspects of modulator performance.
Accordingly, what is needed in the art is an optoelectronic device that does not experience the drawbacks associated with the prior art devices.